1. Field of the Invention
The present invention generally relates to a beam scanning device and an image forming apparatus.
2. Description of the Related Art
General types of image forming apparatuses such as multifunction peripherals having the combined functions of printer/copier/facsimile device may include color image forming apparatuses, for example. Such color image forming apparatuses shine laser beams emitted from a plurality of light sources on four image carriers (e.g., photoconductive drums) arranged in parallel so as to create latent images. The latent images formed on the image carriers are developed with developers (e.g., yellow, magenta, cyan, black toners) for visualization. A transfer material such as a print paper sheet carried on a transfer conveyer belt passes successively through a series of transfer units corresponding to the respective image carriers. The visualized images in the respective colors formed on the respective image carriers are transferred one over another onto the transfer material. The image transferred onto the transfer material is then fused to create a color image.
A beam scanning device (optical writing unit) used in such color image forming apparatuses may include a plurality of light sources, a beam deflecting means to deflect the light beams emitted from the plurality of light sources in two directions symmetrically, and an optical system that is arranged symmetrically on the two sides of the beam deflecting means to guide and focus the light beams on the respective scan surfaces as the light beams are deflected and scanned by the beam deflecting means. Patent Document 1 discloses an optical housing that includes the plurality of light sources, the beam deflecting means, and the optical system.
[Patent Document 1] Japanese Patent Application Publication No. 2002-196269
The optical writing unit described in Patent Document 1 is provided with a light blocking member for blocking reflected/scattered light (flare light) coming from the opposing optical system as the optical systems are arranged on the two sides of the beam deflecting means. Such light blocking member is located around the beam deflecting means inside the housing, and is situated outside the area where the light beams is deflected and scanned by the beam deflecting means.
Such light blocking member for blocking reflected/scattered light (flare light) coming from an optical system is also disclosed in Patent Document 2 and Patent Document 3. Patent Document 2 discloses an image forming apparatus in which a light blocking member is provided at one or more locations of the deflecting unit so as to block a portion of a light beam from a light source as the portion does not contribute to the formation of an image. Patent Document 3 is directed to a multi-beam scanning optical system. There is a need to prevent ghost light reflected by each reflective surface of the polygon mirror from entering the optical path of a correct light beam. Such ghost light is reflected on the adjacent surface of the polygon mirror after coming back along a path parallel to the optical axis of an fθ lens (scan lens). A light blocking plate having a rectangular shape perpendicular to the optical axis of the fθ lens (scan lens) is provided such as to have its tip located in a space enclosed by the optical path of the reflected ghost light and the outline of the scan area of the correct light beam at the scan end opposite the side where the laser beam enters, thereby blocking light outside the scan area of the correct light beam.
[Patent Document 2] Japanese Patent Application Publication No. 2003-255254
[Patent Document 3] Japanese Patent Application Publication No. 2004-29671
Further, Patent Document 4 discloses a scan focus optical system receiving a corresponding deflected light beam incident thereto which is provided on the two sides of the rotating polygon mirror such as to form optical axes substantially parallel to each other. A light blocking means is provided to prevent the reflected errant light coming from one of the two scan focus optical systems from entering the other one of the two scan focus optical systems. This light blocking means blocks the light coming from one of the scan focus optical systems outside the housing of the rotating polygon mirror for the sake of the other one of the scan focus optical systems.
[Patent Document 4] Japanese Patent 3566474
Patent Document 5 is directed to interference between a lens edge and the optical path of a laser beam (incident light) traveling from the optical source unit to the rotating polygon mirror where such interference occurs when the focusing lens system is arranged near the rotating polygon mirror due to the miniaturization of the optical housing or the like. The disclosed scanning optical apparatus avoids such interference with respect to the optical path of the incident light by providing a cut by removing an edge portion of the lens.
[Patent Document 5] Japanese Patent Application Publication No. 2000-267036
In the following, an example of the related-art beam scanning devices will be described with reference to FIG. 11.
In the illustrated beam scanning device, four light sources 101K, 101M, 101C, and 101Y, corresponding to four respective image carriers (black K, magenta M, cyan C, and yellow Y), emit light beams, which are then deflected and scanned by a beam deflector 102 so as to form two pairs of deflective scans on the two respective right/left-hand sides, each pair being comprised of an upper part and a lower part. An fθ lens 103R and fθ lens 103L, which are arranged on the two respective sides, perform fθ correction so as to turn a constant angular velocity scan by the beam deflector 102 into a constant velocity scan on the image carrier surfaces. The light beams of the two pairs scanned in the upper part and lower part are reflected and separated by return mirrors 104Y, 104C, 104M, and 104K for provision to the respective image carrier. Surface-tilt correction lenses 105Y, 105C, 105M, and 105K serve to ensure that the lengths of the optical paths extending to the image carriers become equal to each other.
Further, synchronization detecting sensors 106Y, 106C, 106M, and 106K are provided outside the imaging area on the upstream side of the scan in one-to-one correspondence to the four scans. The synchronization detecting sensors 106Y, 106C, 106M, and 106K serve to synchronize the turn-on timing of the light sources 101Y, 101C, 101M, and 101K, respectively.
The light sources 101Y, 101C, 101M, and 101K, the beam deflector 102, the fθ lenses 103R and 103L, the surface-tilt correction lenses 105Y, 105C, 105M, and 105K, the return mirrors 104Y, 104C,104M, and 104K, and the synchronization detecting sensors 106Y, 106C, 160M, and 106K are accommodated in an optical housing 110.
The synchronization detecting paths directed toward the synchronization detecting sensors pass through the effective scan area of the lenses, and reach the synchronization detecting sensors with a predetermined beam diameter and power. The light sources are lit up before reaching the synchronization detecting sensors so as to ensure that the light beams are received and detected by the sensors even if the installed positions of the synchronization detecting sensors are not aligned.
In recent years, higher image quality has been pursued. Due to this, control to achieve a constant power of the light sources inside the scan area is performed before the light beams reach the synchronization detecting sensors. Some image forming apparatuses require a time allotted for such control.
In image forming apparatuses, costly lenses may be replaced with general-purpose lenses so as to allow the sharing of these lenses between different apparatuses, thereby reducing the size and cost. Because of this, the effective scan areas of the lenses are set to a required minimum
Further, the time required for synchronization-purpose pre-lighting is determined according to the control circuit and the light sources such as LDs. In the beam scanning devices for providing high-speed scans as used in high-speed image forming apparatuses, a scan area scanned per constant time period increases, resulting in a need for an increase in the area of the synchronization-purpose pre-lighting.
As a result, the synchronization-purpose pre-lit light L1 and L2 (see FIG. 11) falls outside the effective lens area. If there is a discontinuous surface outside the effective lens area, the flare light of such a discontinuous surface may enter the imaging area on the image carriers, resulting in the problem of an abnormal image.
A plastic lens made through plastic molding may be used as an fθ lens for the purpose of cost reduction. When plastic molding is used, it is typical to pour a resin in the longitudinal direction of the lens due to reasons associated with the use of plastic molding. Since the shape of the inlet (gate) through which the resin is poured is different from the shape of the lens, the resulting lens inevitably has a portion with foreign shape. A discontinuous surface is thus created at the boundary between the lens surface and the portion having foreign shape outside the effective lens area. This gives rise to the problem that the flare light of the discontinuous surface enters the imaging area on the image carriers to create an abnormal image.
In high-speed machines (image forming apparatuses), further, the beam deflector may be required to have a good heat releasing performance with respect to the generated heat and a good anti-vibration performance against vibration caused by external disturbance. In such a case, the optical housing for accommodating the light sources and the beam deflector is implemented as a metal optical housing rather than as a resin optical housing.
In general, metal-molded products are not shaped well compared with resin-molded products, and it is difficult to create a product having thin thickness. Further, if positional accuracy is required, an additional step of cutting and scraping may be necessary. Moreover, a metal surface has a relatively higher reflectivity compared with a resin surface. If a rib formed as part of the optical housing for the purpose of blocking flare light, thus, reflection occurs on the surface of the rib, resulting in the reflective light entering the imaging area.
Accordingly, there is a need for a beam scanning device which can block flare light with sufficient accuracy so as to prevent a drop in image quality while using a metal housing to ensure good heat releasing performance and anti-vibration performance. Further, there is a need for an image forming apparatus provided with such a beam scanning device.